Recent work has led to the hypothesis that the percepts generated by visual cortex are determined stochastically according to the success (or failure) of past behaviors in response to inevitably ambiguous retinal images. So far, the basis for this idea is indirect: psychophysical evidence shows that the images seen in response to a wide variety of stimuli accord with the probability distributions of the possible sources of the corresponding retinal image. These findings imply that the primary role of the visual cortical processing -- and by inference the role of sensory cortices generally -- is to instantiate and process the probability distributions of the possible stimulus sources. Here we propose to initiate a direct exploration of this novel concept of cortical function by taking advantage of the ease (and accuracy) with which visuotopic and orientation maps can be determined in the primary visual cortex (V1) by optically imaging patterns of cortical activity. Using this method, we propose to examine how geometrical target angles that elicit very different perceptions as a function of context are represented in an experimental animal (the tree shrew to begin with). The project entails three Specific Aims: 1) To determine with optical imaging of 'intrinsic signal' how simple angular stimuli are represented in VI of anesthetized animals, using both topographic and orientation maps as indices; 2) To determine the cortical representations of topography and orientation elicited by the same stimuli in contexts that markedly change the probability distributions of the possible sources (and thus perception); and 3) To examine whether awake behaving animals, much as human subjects, see the same geometrical stimuli differently as a function of contextual statistics. These experiments will show whether visuotopic and orientation maps represent image features (i.e., location in space and line orientation), as has long been thought, or whether, as we suspect, they represent the probability distributions of the possible sources of visual stimuli. Knowledge that the primary visual cortex operates in this probabilistic way would demand a reinterpretation of present structural and functional information about V1, and would strongly encourage further exploration of other sensory cortices using this framework.